This invention relates generally to a method for the separation of halogenated compounds, and, more specifically, to a method of removing the toxic impurity chlorofluoromethane (HCFC-31) from a product stream of difluoromethane (HFC-32).
Historically, chlorofluorocarbons have been widely used in various capacities such as refrigerants, foam blowing agents, cleaning solvents and propellants for aerosol sprays. In recent years, however, there has been pressure to avoid their use due to their adverse effect on the ozone layer and their contribution to global warming. Consequently, attempts are underway to find suitable replacements which are environmentally acceptable. The search for suitable replacements has centered generally on hydrofluorocarbons (HFCs) which do not contain chlorine. The hydrofluorocarbon difluoromethane (HFC-32) is of particular interest as one such replacement. Difluoromethane has an ozone depletion potential (ODP) of zero and a very low global warming potential (GWP).
A widely-used method for preparing hydrofluorocarbons involves the fluorination of chlorinated starting materials. Unfortunately, fluorination of chlorinated staring materials usually results in the formation of unwanted, chlorinated by-products. For example, production of HFC-32 tends to produce a variety of chlorinated methane by-products including chlorodifluoromethane (HCFC-22), dichlorodifluoromethane (CFC-12), and chlorofluoromethane (HCFC-31). While distillation effectively removes many chlorinated impurities from an HFC product stream, some chlorinated impurities, particularly HCFC-31, cannot readily be removed through conventional distillation. Nevertheless, HCFC-31 must be removed to extremely low levels, for example, below 10 ppm, because it is highly toxic and tends to react with the desired HFC product.
Therefore, there is a need to remove chlorinated methane impurities, particularly HCFC-31, from a product stream more effectively then through distillation. The present invention fulfills this need among others.
The present invention relates to the identification of a commercially-available polymer adsorbent that removes chlorinated methane impurities from a product stream. Although polymeric absorbents are known to remove organics from air and water (see, e.g., Dow Chemical Company, Polymeric Adsorbent XUS 43493, Technical Bulletin 3.03 (hereby incorporated by reference)), it has been found unexpectantly that the adsorbent of the present invention is particularly suitable for selectively adsorbing chlorinated methanes over halogenated compounds. In particular, the adsorbent of the present invention adsorbs chlorinated methanes, such as HCFC-31, but not hydrofluorocarbons, such as HFC-32.
One aspect of the present invention is a process of using a polymer adsorbent to remove a chlorinated methane impurity from an impure product stream comprising a halogenated compound other than the chlorinated methane impurity. In a preferred embodiment, the polymer adsorbent has a pore size distribution characterized by a cumulative porosity as a function of the log of pore diameter greater than that of activated carbon. In another preferred embodiment, the adsorbent comprises a matrix of at least one cross-linked styrenic polymer having a total porosity of at least about 0.8 cc/g, an average pore diameter of about 30 to about 60 xc3x85, and a BET surface area of at least about 900 m2/g.
The process of the present invention has been found to be particularity effective in adsorbing a chlorinated methane impurity having the formula:
CHwClyXzxe2x80x83xe2x80x83(1) 
wherein: each X is an independently selected halogen; yxe2x89xa71 and w+y+z=4.
Preferably X is fluorine. In a more preferred embodiment, the chlorinated methane impurity is selected from the group consisting of chlorofluoromethane (HCFC-31), dichloromethane (HCC-40), chlorodifluoromethane (HCFC-22), chlorotrifluoromethane (CFC-13), dichlorodifluoromethane (CFC-12) and combinations of two or more thereof. In the most preferred embodiment of the invention, the chlorinated methane impurity is HCFC-31.
In a preferred embodiment, separation is effected between a chlorinated methane of formula (1) and a halogenated compound having the following formula:
CnHmClpXxe2x80x2kxe2x80x83xe2x80x83(2) 
wherein:
each Xxe2x80x2 is an independently selected halogen other than chlorine; and
n, m, p, and k are integers with the provisos that 1xe2x89xa7nxe2x89xa710; n greater than p; kxe2x89xa71; and 2n+2=m+p+k.
More preferably, nxe2x89xa63, p=0, and Xxe2x80x2 is fluorine, and, even more preferably, n=1. In the most preferred embodiment, the product stream comprises HFC-32.
It is believed that pore distribution of the adsorbent may play a significant role in the selectivity described above. (The scope of the invention, however, should not be limited by any particular theory of adsorption). As used herein, xe2x80x9cpore distributionxe2x80x9d is a linear relationship between cumulative porosity and the log of the pore diameter. The preferred adsorbent of the present invention has pore distribution characterized by a higher cumulative porosity as a function of the log of pore diameter than that of activated carbon. In a more preferred embodiment, the pore size distribution is characterized by a cumulative porosity as a function of the log of pore diameter of no less than about 0.43 cc/g. Still more preferably, the cumulative porosity as a function of the log of pore diameter of no less than about 0.45 cc/g. The linear relationship of cumulative porosity to the log of pore diameter can vary, although a portion of the relationship is characterized by an exponential increase in cumulative porosity.
In a preferred embodiment, the absorbent comprises a matrix of at least one cross-linked styrenic polymer having a total porosity of at least about 0.8 cc/g, an average pore diameter of about 30 to about 60 xc3x85, and a BET surface area of at least about 900 m2/g. More preferably, the total porosity is about 1.1 cc/g, average pore diameter is about 35 to about 55 xc3x85, and BET surface area is at least about 1000 m2/g. Still more preferably, the total porosity is about 1.1 to about 1.2 cc/g, the average pore diameter is about 40 to about 50 xc3x85, and the BET surface area is at least about 1100 m2/g. In the most preferred embodiment, the total porosity is about 1.16 cc/g, the average pore diameter is about 46 xc3x85, and the BET surface area is about 1100 m2/g.
It has been found that polymeric adsorbents having relatively low moisture content tend to outperform equivalent adsorbents having relatively high moisture content. Accordingly, in a preferred embodiment, the moisture content is no greater than about 30% by weight, more preferably, no greater than about 10% by weight, and, even more preferably, no greater than about 5% by weight.
The configuration of the units of adsorbent may vary providing that the physical parameters above are met. It has been found, however, that spherical beads achieve the desired results. In a preferred embodiment, the beads have a diameter from about 10 to about 70 mesh, and, more preferably, from about 20 to about 50 mesh. Suitable results have been obtained using an adsorbent having an apparent density of about 0.20 to about 0.80 g/cc. Preferably, the apparent density is about 0.30 to about 0.70 g/cc, and, more preferably, about 0.34 g/cc.
Particular preferred and commercially-available polymeric adsorbents useful in the present invention includes DOWEX OPTIPORE 493 Series (available through Dow Chemical, Midland, Mich.), especially V493, which is described in detail in Dowex Optiore Adsorbents, Fluidized Properties of Dow Polymeric Adsorbent, Form No. 177-01731-597ORP (May 1997), herein incorporated by reference.
In the process of the invention, the product stream is contacted with the zeolite by passing the product stream over a fixed bed of polymeric absorbent in either the liquid or vapor phase. It has been found, however, that more effective removal of chlorinated methane impurities is achieved using a vapor-phase product stream. The bed should be packed tightly to ensure that very little, if any, vapor stream xe2x80x9cbreaks throughxe2x80x9d and passes through the bed without contacting the adsorbent sufficiently to promote adsorption. Selection of the pellet size and bed shape may be varied within a broad range and may be determined according to known principles, and, particularly, to provide the preferred densities described above. Various other techniques known in the art also may be used for contacting the product stream with the polymeric absorbent particles, including, for example, fluidized or moving beds of polymeric absorbent particles. Selection of the particle size and bed shape may be varied within a broad range and may be determined according to known principles, and, particularly, to provide the preferred pore distribution, porosity and/or surface area as described above.
The hourly space velocity of the product stream over the polymeric absorbent may be varied within a wide range. Generally, the product stream is passed over the active carbon with a gas hourly space velocity of about 5 to about 1000 hxe2x88x921, and preferably with a gas hourly space velocity of about 10 to about 500 hxe2x88x921, although the gas hourly space velocity may be much greater or much lower than this if desired. A corresponding liquid hourly space velocity for liquid phase operation is about 1 to about 30 hxe2x88x921, and, again, this velocity may be more or less if desired.
The conditions under which the process of the present invention is conducted may be varied widely and generally depend upon the equipment available. Typically, the temperature at which the vapor phase process is conducted is between about xe2x88x9250 and about 100xc2x0 C., more conveniently, between about 0 and about 50xc2x0 C., and even more conveniently at about room temperature. The pressure will be dependent to some extent upon whether liquid or vapor phase contacting is chosen and the operation temperature, although an operation pressure between about 0.1 and about 30 bar is generally suitable. Preferably, the process is conducted at about atmospheric pressure or slightly below to avoid the use of specialized equipment.
The bed of polymeric absorbent will require regeneration to desorb the chlorinated impurity when its absorption capacity has been filled. Regeneration may be performed by passing a gas stream, typically nitrogen or air, over the bed of polymeric absorbent at elevated temperature, for example, from about 50 to about 150xc2x0 C., and preferably below about 100xc2x0 C.
According to the process of the present invention, chlorinated methane impurities can be effectively removed from a product stream with high selectivity. The process of the present invention is particularly well suited for removing HCFC-31 from a product stream comprising HFC-32. For example, it has been found that the process of the present invention can be used to purify a vaporized product stream having a space velocity of no greater than about 100 hrxe2x88x921 in a tube packed with adsorbent of the present invention over a period of no greater than about 4 hours to result in a purified product stream containing less than 10 ppm of HCFC-31.
The following examples serve to illustrate the invention: